Bump gatsby from 2.22.12 to 2.22.15

Bumps [gatsby](https://github.com/gatsbyjs/gatsby) from 2.22.12 to 2.22.15.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/compare/gatsby@2.22.12...gatsby@2.22.15)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -7,7 +7,7 @@
     "axios": "^0.19.2",
     "bytes": "^3.1.0",
     "classnames": "^2.2.6",
-    "gatsby": "^2.22.12",
+    "gatsby": "^2.22.15",
     "gatsby-image": "^2.4.5",
     "gatsby-plugin-manifest": "^2.4.9",
     "gatsby-plugin-matomo": "^0.8.3",